1. Field of the Invention
The present invention relates generally to a communication system for movable bodies or slave units. Communication signals from a master unit are sent over power lines laid along a rail on which the movable bodies run. More particularly, the present invention relates to a detachable antenna unit in which master communication antennas are located.
2. Description of the Related Art
There is a monorail type conveying apparatus in which carriages run along a rail, which is laid on the ceiling of a factory or a warehouse, to convey loads between stations in the factory or warehouse. Japanese Unexamined Patent Publication No. 6-153305 discloses one example of a power supply system for such a conveying apparatus. According to the power supply system, each carriage has a pickup coil, which faces power lines laid along the rail in a contactless state. The pickup coil acquires induced electromotive force as power from the alternating current that flows through the power lines.
The power supply system disclosed in Japanese Unexamined Patent Publication No. 6-153305 will more specifically be described referring to FIGS. 8 and 9. As shown in FIG. 8, two power lines 83 are supported on a rail 81 by support members 82. An alternating current (a high-frequency alternating current of, for example, 200 V) flows through each power line 83. Each carriage acquires power from a pickup device 84 provided in a contactless state facing the two power lines 83. The pickup device 84 includes a core 85 having an approximately E-shaped cross section, and a coil 86 wound around the center projection, 85a, of the core 85.
The carriage communicates with a fixed station provided on the ground, and its operation is controlled based on an instruction signal from the fixed station. When the fixed station, which is known as a parent station or master unit, and the carriage, which is known as a child station or slave unit, communicate with each other via radio signals, cross-talk may occur. Normally, therefore, trolley lines laid along a rail are used to ensure communications between the fixed station and the carriage. When trolley lines are used, however, a collector brush provided on the carriage to contact the trolley lines wears out easily. Further, the laying the trolley lines in addition to the power lines increases the line laying work.
To overcome this shortcoming, a communication method using the power lines has been proposed. In the proposed communication method, a signal for communications is superimposed on the AC that flows through the power lines. The communications between the fixed station and the carriage are accomplished via a pickup coil antenna, which is provided on the carriage.
To improve the communication reliability, it is necessary to suppress as much as possible the attenuation of a signal sent onto the associated power line from the communication antenna. Since antennas cannot be directly wound on the power lines along which the carriage moves, antennas are provided close to the power lines. Since antennas can be fixed to the power lines at the fixed station, by contrast, a communication antenna 87 on the fixed station should preferably be directly wound on each power line 83 as shown in FIG. 9.
Direct winding of the communication antenna 87 around the power line 83 as shown in FIG. 9, however, requires winding a coil midway along the lengthy power line 83. Because, generally speaking, the power line 83 is very long and heavy, handling the power line is not easy. Therefore, the work of winding the communication antenna 87 is very troublesome and inefficient. Further, it is very difficult *to prepare the antenna 87 in the field as designed, and the resulting antenna 87 does not have high quality, which may reduce the reliability of communications.